1. Field of the Invention
The present invention relates to a method for eliminating dissolved gases, especially dissolved oxygen, from liquid products that need to be deoxidized in terms of retention of product quality, such as foaming alcoholic drinks, carbonated drinks, or other like drinks, and medical supplies, and cosmetics.
2. Description of the Prior Art
As is well known, dissolved oxygen in a liquid is considered to be the main factor behind the oxidation. When the liquid itself is a product, the dissolved oxygen may cause components in the liquid to be deteriorated. When the liquid is designed for use of washing or like applications, the dissolved oxygen may cause the washing object to be oxidized. For example, in the case of liquors such as refined sake, wine, and distilled spirits, soft drinks, fruit drinks, and the like, there may occur deterioration in flavor, discoloration, and the like due to the effect of oxygen remaining dissolved in the drink. To prevent such phenomena, it is desirable to eliminate the dissolved gases including oxygen from the liquid product. Available methods for eliminating the dissolved gases are the aeration process (gas substitution process), the heat deaeration process, the chemical deaeration process, and the like.
Among the above-listed deaeration processes, the heat deaeration process is such that a liquid to be deaerated is heated in an open system whereby dissolved gases are discharged and eliminated. Accordingly, there is a possibility that oxygen may be redissolved in the liquid due to careless operation or malfunction of equipment. Further, the method cannot be applied to such liquids as may be deteriorated in quality by heat. As for the chemical deaeration process, since chemicals such as deoxidizing agents are used, the flavor of the product may be spoiled by the chemicals, and moreover safety concerns are involved. Thus, for liquid products as drinks such as mentioned above, the aeration process and the vacuum aeration process have been adopted.
In the aeration process (gas substitution process), a high-purity gas (e.g., nitrogen gas, carbon dioxide gas) other than oxygen is blown into a liquid to be deaerated within an aeration tank, so that the high-purity gas and the liquid are brought into direct contact with each other, whereby the oxygen partial pressure in the liquid is reduced. Thus, the dissolved oxygen is eliminated in a physical fashion.
The vacuum deaeration process is such that the atmosphere in contact with a liquid to be deaerated is evacuated by using a deaeration tower, so that gas partial pressure is reduced, whereby dissolved gases in the liquid are eliminated. In recent years, as one type of the vacuum deaeration process, the membrane type vacuum deaeration process has been put into practical use, in which a gas-permeable membrane (hydrophobic polymer membrane) that allows not water but only gas to pass therethrough is used and dissolved oxygen in the water is separated to a vapor phase under reduced pressure via the gas-permeable membrane.
However, as in foaming liquors such as beer and sparkling wine or in carbonated drinks, in the case of liquids that need to keep a particular gas (carbon dioxide gas in the above liquids) dissolved therein, the aforementioned vacuum deaeration process and heat deaeration process would cause not only dissolved oxygen but carbon dioxide gas, which is a crucial element in terms of flavor as such types of products, to be eliminated as well. As a result, there would arise the need of reading carbon dioxide gas later, which involves a high cost for carbon dioxide gas to be readed. Consequently, for the deaeration process for such types of liquids, the aeration process has been adopted to accomplish deoxidization by using carbon dioxide gas as the high-purity gas.
As stated above, in order to eliminate dissolved gases including oxygen from liquid products containing a particular gas, as in foaming liquors and carbonated drinks (carbon dioxide gas in these drinks), aeration using carbon dioxide gas has been applied. However, to reduce the concentration of dissolved gases in the liquid to low level, it is necessary to use an extremely high purity carbon dioxide gas. Besides, various measures should be taken, for example, foams of carbon dioxide gas to be blown into the liquid need to be sized as small as possible, so that the contact area between carbon dioxide gas and the liquid is maximized. Moreover, much time is taken. More specifically, for example, even if beer is aerated with carbon dioxide gas for 40 min, it is difficult to control the concentration of dissolved oxygen to not more than 1 ppm.
Further, due to structural restrictions in the aeration equipment, the aeration is performed under a pressure higher than liquid pressure. Therefore, high liquid pressure makes it difficult to eliminate dissolved gases, as another problem.
Besides, since the aeration process is performed in batch processing, a large amount of gas for aeration is necessitated. In addition, a large amount of foams is generated as a result of aeration, so that the aeration tank is reduced in its effective capacity. Therefore, a large-scale aeration tank is necessitated.
Still further, when beer is aerated, for example, with carbon dioxide gas in such a tank, foams of beer would go in the and out through the air vent in upper part of the aeration tank. This causes a possibility of bacterial poisoning from outside the tank, which is undesirable also from a hygienic point of view.